The length of transistor gates of currently available CMOS integrated circuits is 0.8 .mu.m. With reduction in size of CMOS integrated circuit devices, the length of transistor gates tends to be smaller, and presently CMOS integrated circuits having transistor gate lengths of 0.6 .mu.m are in the development.
For a transistor having a transistor length of 0.8 .mu.m, a transistor gate is operable with 5 volts power-supply voltage applied thereto, however, for a transistor having a transistor length of 0.6 .mu.m, the power-supply voltage of such a transistor is expected to be reduced to a value between 2.7 V and 3.3 V for assuring pressure proof. As transistor gates become even smaller, for example if the gate lengths are to be in the range of 0.3 to 0.2 .mu.m, the power-supply voltage applied to the transistor gates will be reduced even more.
If the transistor gate length is shortened to the range of 0.2 to 0.3 .mu.m, the power-supply voltage is expected to be approximately 1.5 volts. Thus, when a very minute transistor gate having a gate length of 0.6 .mu.m or less is to be manufactured, it is impossible to avoid an approximately 20% variation in actual gate length. This variation between the desired gate length and the actually manufactured transistor gate length is important. For example, if it is desired to make a gate with a gate length of 0.3 .mu.m, and the actually manufactured transistor gate length is 0.25 .mu.m or 0.35 .mu.m, problems arise in that operating speed and value of the power-supply voltage of transistor gates are noticeably different than expected.